Re: template instantiation g++ 4.0x

[Tom H]

Paul Pluzhnikov wrote:
> Tom Hilinski <tom@nowhere.net> writes:
>
> > I'm unable to get template code that builds with g++ 3.4.x to link
> > successfully in g++ 4.0.x
<snip>
> What you are doing is non-portable: when the instantion requests below
> are seen by g++, it has not seen the body of ATemplate<T>::GetValue()
> yet, and therefore can't actually instantiate anything.
>
> > template class ATemplate<int>;
> > template class ATemplate<float>;
>
> Here is a fix:
>
>   $ cat ATemplate.cpp
>   #include "ATemplate.h"
>   template <class T> T ATemplate<T>::GetValue () const { return n; }
>   template class ATemplate<int>;
>   template class ATemplate<float>;
>
> Your ATemplateInst.h serves no apparent purpose anyway.
>
> Cheers,

Thanks for your quick reply. My understanding of how g++ handles 
template instantiation in 3.4 agrees with the code example. From the 
manual (for both 3.4 and 4.0),the default compiler action is:

"...the compiler emits template instances in each translation unit that 
uses them, and the linker collapses them together."

So, ATemplate.cpp will have int and float versions of member GetValue. 
The header file ATemplateInst.h (which mimics the real system even if it 
isn't needed in this example) forces the instantiations. The linker (or 
collect2) should find the GetValue instances in ATemplate.o. This is 
what version 3.4 does, and 4.0 does not.

Looking at the symbols in the .o files shows that 4.0 is not generating 
instances of GetValue:

   nm -C ATemplate.o

g++ 3.3:

00000000 W ATemplate<float>::ATemplate[in-charge]()
00000000 W ATemplate<float>::ATemplate[not-in-charge]()
00000000 W ATemplate<int>::ATemplate[in-charge]()
00000000 W ATemplate<int>::ATemplate[not-in-charge]()
00000000 W ATemplate<float>::GetValue() const
00000000 W ATemplate<int>::GetValue() const

g++ 4.0:

0000000000000000 W ATemplate<float>::ATemplate()
0000000000000000 W ATemplate<float>::ATemplate()
0000000000000000 W ATemplate<int>::ATemplate()
0000000000000000 W ATemplate<int>::ATemplate()
                 U __gxx_personality_v0


Clearly the behavior of 4.0 in regards to template instantiation has 
changed from version 3.4.x. So which version is "correct"?  Is it 
possible to force 4.0 to revert its actions to that of 3.4?

...Tom